Chemicals that cause cancer have a variety of molecular structures and include complex hydrocarbons, aromatic amines, and certain metals, drugs, hormones, and naturally occurring chemicals in molds and plants. Many nitrosaminesÑsimple organic oxides of nitrogenÑare carcinogenic and may be produced within the human body. Hydrocarbons and nitrosamines are components of cigarette smoke and may be carcinogenic agents contributing to lung cancer. Certain aromatic amines, especially 2-naphthylamine, were used in the dye industry for dyeing cloth and other materials until it became apparent that this chemical caused a high incidence of bladder cancer in workers. Another industrial chemical gas, vinyl chloride, has been implicated as the causative agent of sarcoma of blood vessels in the livers of exposed workers.
Several drugs used to treat cancer are also carcinogenic; although these chemicals are used to break DNA strands of cancer cells, thereby killing the cells, this same property causes the agent to induce cancer in normal cells. High levels of estrogensÑa group of female hormonesÑadministered to women after menopause result in an increased incidence of cancer of the uterus. Aflatoxin BN, a complex molecule produced by strains of the mold Aspergillus, causes various cancers, particularly liver cancer. Certain salts containing arsenic are probably related to cancer of the skin and liver.
That such a variety of apparently unrelated chemical structures can induce cancer by a common mechanism was indicated by the work in the 1960s of James and Elizabeth Miller, who demonstrated that chemical carcinogens, in order to exert their carcinogenic effect, must first be metabolized to an active, or "ultimate," form capable of reacting directly with cellular macromolecules, especially DNA. This and other research has strongly suggested that the formation of a cancer from a normal cell results from a chemical or structural alteration in the DNA of that cell.